As mobile device technology continues to develop and demand therefor continues to increase, demand for secondary batteries as energy sources is rapidly increasing. Among these secondary batteries, lithium secondary batteries, which have high energy density and operating voltage, long cycle lifespan, and low self-discharge rate, are commercially available and widely used.
In addition, as interest in environmental problems is increasing, research into electric vehicles (EVs) and hybrid EVs (HEVs) that can replace vehicles using fossil fuels, such as gasoline vehicles, diesel vehicles, and the like, which are one of the main causes of air pollution, is actively underway. As a power source of EVs, HEVs, and the like, a nickel metal-hydride secondary battery is mainly used. However, research into lithium secondary batteries having high energy density and high discharge voltage is actively underway and some lithium secondary batteries are commercially available.
Lithium ion secondary batteries used in conventional small batteries generally use a lithium cobalt composite oxide having a layered structure as a cathode and a graphite material as an anode. However, in the case of the lithium cobalt composite oxide, cobalt, a main constitution element, is very expensive and the lithium cobalt composite oxide is not sufficiently stable for application to electric vehicles. Therefore, as cathode of a lithium ion battery for electric vehicles, a lithium manganese composite oxide composed of manganese, which is cheap and has superior stability, and having a spinel structure may be proper.
However, in the case of the lithium manganese composite oxide, manganese is eluted to an electrolyte solution by an electrolyte solution during high-temperature storage and, thereby, degrading battery characteristics. Therefore, solutions to prevent this problem are required. In addition, the lithium manganese composite oxide has a drawback that capacity per unit weight is small, when compared to lithium cobalt composite oxides or lithium nickel composite oxides, and thereby increase of capacity per battery weight is limited. Accordingly, when a battery is designed such that the limitation is improved, the battery may be commercialized as a power source of electric vehicles.
Such cathode active materials during charging may reduce stability of a battery cell through exothermic reaction accompanying degradation of a surface structure and drastic structural collapse. Thermal stability is associated with interfacial stability between an electrolyte and a cathode active material. Accordingly, most patent literature uses general coating methods to improve surface stability and disclose a plurality of different coating methods.
When the prior art is taken together, there are two type coating methods, namely, cathode ion coating and anode ion coating. Al2O3 coating is a representative example of cathode ion coating and an example of anode ion coating includes fluoride, phosphate, and silicate coating. Here, fluoride coating is the most preferable in that the fluoride coating is thermodynamically very stable due to formation of a protective film of LiF and may provide satisfactory stability at high temperature and voltage, since the fluoride coating does not react with an electrolyte. Meanwhile, coating type may be classified into inorganic coating and organic coating, and polymer coating as an example of the organic coating may provide an elastic coating.
However, the conventional coating methods cannot provide satisfactory battery cell stability. As well as, thin and dense LiF films cannot be provided due to a high melting point and poor wetting properties of LiF and polymer coating may deteriorate overall properties of a lithium secondary battery due to problems such as poor electrical conductivity and lithium migration.
Therefore, there is an urgent need for coating technology which may improve stability by protecting a surface of a cathode active material without deterioration of battery characteristics and improve overall properties of a battery.